Pay for Inclusion (Paid Inclusion or PFI) is defined as
“the act of paying a search engine or directory to include your web site in their index. Paying this fee does not affect the placement of a web site in search results, it simply assures that the web site will be visited and indexed within a specified time frame” (Strategic Ranking).
Paid inclusion does not guarantee a top position in the search results and without doing search engine optimization, pages that have been paid for to be included in the index will not appear high in the search results. Therefore, paid inclusion is not a suitable option for un-optimized websites.
PFI used to be a popular method of getting pages indexed in particular search engines fast. However, at the time of writing, many members of the webmaster community are predicting its extinction in the near future (Ulbrich 2004, WebmasterWorld.com a). Because of this fact, it is pointless to go into too much detail about PFI. Paid Inclusion is currently offered by Inktomi, Overture (Yahoo!) and Lycos.